Which of the following represents irrational numbers?

Irrational numbers are defined as numbers that cannot be expressed as a simple fraction, meaning they cannot be represented as the ratio of two integers. They have non-repeating and non-terminating decimal expansions.

The choice of √2 is correct since it is known to be an irrational number. Its decimal expansion is approximately 1.41421356..., continuing infinitely without repeating. This property makes it impossible to express √2 as a fraction of two integers.

The other options represent rational numbers. For example, 0.5 can be expressed as 1/2, making it a rational number. Similarly, 1/3 is explicitly a fraction and hence rational. Lastly, -7, being an integer, can also be expressed as -7/1, fulfilling the criteria of being a rational number. Therefore, the only choice that fits the definition of an irrational number is √2.